The Spirit Realm

Well, correct me if I misinterpret this, but the reason you're wanting to box a knight is because your friend's Monk can't tank that well. Is that it?

Frankly, the solution isn't boxing a knight, it's getting your friend to restart an SK and sideline the Monk.

But, if your friend won't budge, then you have two options.

Option 1) box a tank.
Option 2) box a support class to keep the MNK on his feet despite limited tanking.

If you choose Option 1, then take the SK. An SK will bring snare and DOT camp utility and will hold aggro against the MNK's massive DPS.

If you choose Option 2, the spectrum of choices gets much broader, but i'd recommend a DRU box. A DRU would bring everything to the table that your duo is now missing. Two healers and buffers means the MNK stays on his feet dealing damage. The DRU can also be a significant DPS addition as well. Dependable snare enabled, long-lasting root enabled, ports enabled, mana regen enabled, DOT camps enabled.

If you choose option 1, your tanking issue is solved, but you will not add much DPS or group support. (You will add both, just not much of both.) If you choose option 2, your tanking issue is still there and will limit you as to what content you can get to, but within the scope of that content, your little trio will be hard to kill.

Getting more AC on the MNK and maxing his defensive AAs will help considerably. Just 100 more raw AC makes a huge difference in damage taken. There are some excellent threads over on the Steel Warrior's forum that demonstrate this very well. So your friend can do some things to improve his durability. AAs are the place to start. If he hasn't maxed his defensive AAs then light a fire under him.

Again, I want to emphasize that the best solution is retiring the MNK in favor of an SK, but that depends on your friend.

Regards.

Personally, I would just stick with what you have. You have DPS/slowing/pulling/healing (depending on content and AAs) covered. Finding a tank will get easier as you level up. I bet that now you are grouping with people's alts and they probably do not have a clue how to play the class. As you get to higher level, you will play with people that have tanks as a main.
For the amount of time you would spend creating and leveling your pally, you could get a significant amount of healing AAs for your shaman and defensive AAs for the monk. Focus on getting the Monk better gear (as suggested by Sam) and you will be better off. That is the nice thing about shaman, they are not as gear dependant as many classes.